vortex.c

#include <stdio.h>
#include <limits.h>

#define Start (1)
#define Iterations (sizeof(unsigned long long)*8)

int sum_digits(unsigned long long i) {
	if (i<=9) return i;
	static char _buf[512] = {0};
	char *p = &_buf[0];
	int sum=0;
	snprintf(&_buf[0], sizeof(_buf), "%llu", i);
	while (*p)
		sum+=(*p++-'0');
	return sum_digits(sum);
}

int main(void) {
	for (unsigned long long x = Start, iter = 0; iter < Iterations; ++iter, x*=2)
		printf("%d\n", sum_digits(x));
	return 0;
}